This invention relates to improvements in mobile equipment having hydraulic transmission systems and hydraulically-powered accessory systems.
In the past, such mobile equipment have utilized a hydraulic transmission system usually comprising one or more variable displacement hydrostatic pumps powering one or more fixed or variable displacement hydrostatic motors through a closed, recirculating fluid conduit loop or loops. The variable displacement pumps are driven by the vehicle engine, and the hydrostatic motors drive wheels or tracks as the case may be. Each such hydrostatic pump conventionally also includes a fixed displacement charge pump also driven by the engine, to make up any leakage in the closed loop circuit and thereby prevent any cavitation in the system. Such charge pumps are designed to produce a volumetric flow much in excess of the needs of the hydraulic transmission system, such excess normally being exhausted from the system through a pump case drain to the main hydraulic fluid reservoir,from which the charge pump draws its fluid through a sump line and filter.
Such mobile vehicles, especially those used for industrial purposes such as excavating equipment, cranes and forklift trucks, usually also include extensive hydraulic-powered accessory systems such as hydraulic cylinder-operated excavating blades and buckets, crane booms, lifting masts and so forth. These accessories are conventionally powered by pressurized hydraulic fluid received from a separate engine-driven pump, normally of the fixed displacement type, which draws its hydraulic fluid through a separate sump line and filter from the same central fluid reservoir from which the hydraulic transmission system draws its fluid, and returns the fluid to the sump through a separate sump return line.
The above-described conventional hydraulic transmission and hydraulic accessory systems are unduly wasteful in several respects. The need for separate filters, separate sump lines and separate sump return lines for the hydraulic transmission and hydraulic accessory system respectively requires costly additional capital investment in the equipment and occupies valuable space in mobile machines where space is often times at a premium. Furthermore the fluid returned to the sump from the hydraulic transmission system in many cases is at a high temperature level, thereby requiring an oil cooler in the circuit which requires additional cost and space. In addition, the highly variable loads placed upon the accessory system pump, coupled with that pump's need to draw fluid directly from the sump at variable speeds corresponding to those of the engine which drives the pump, in many cases causes the pump to cavitate by virtue of its inability to draw a sufficient volumetric flow of fluid at high pump speed.